1. Field of the Invention
The present invention relates to an ultrasonic medical treatment apparatus for applying a medical treatment to a tumor and the like within a living body by using ultrasonic waves.
2. Description of the Background Art
In recent years, a type of medical treatment called MIT (Minimum Invasive Treatment) has been attracting attentions in various fields of the medicine.
One example of this MIT is a lithotriptor apparatus for applying a lithotripsy treatment to a calculus non-invasively, which has revolutionized medical treatment of the urological calculus. For this lithotriptor apparatus, many different types of the intense ultrasonic wave generation source are available, including an underwater discharge type, an electromagnetic induction type, a micro-explosion type, and a piezoelectric type. Among these, the piezoelectric type has many advantageous features such as that the focal point is small, that no articles of consumption is required, that the pressure of the intense ultrasonic waves can be controlled freely, and that the focal point position can be controlled freely by a phase control of driving voltages to be applied to a plurality of piezoelectric transducer elements, in spite of a drawback that the pressure of the intense ultrasonic waves is rather small. (See Japanese Patent Application Laid Open No. 60-145131 and U.S. Pat. No. 4,526,168 for further details.)
On the other hand, this MIT is also attracting attentions in the field of the cancer treatment. In particular, in a case of a cancer, its medical treatment is still largely relying on the surgical operations today and there are many incidences of severely damaging an original function and/or an outward appearance of an organ with a cancer, such that a heavy burden would be left on the patient even if it is possible to prolong a life of a patient. In view of such current situations, there are strong demands in this field for a development of less invasive medical treatment method and apparatus taking the QOL (Quality Of Life) into account properly.
In such a recent trend, the so called hyperthermia treatment method has been attracting large attentions as a method of medical treatment of a malignant tumor, i.e. a cancer. This hyperthermia treatment method is a medical treatment method in which cancer cells are selectively killed by warming and maintaining a diseased portion at a temperature over 42.5.degree. C., utilizing different heat sensitivities of tumor tissues and normal tissues. As for a scheme of warming, a scheme using the electromagnetic waves such as microwaves has been developed first, but in this scheme, it is difficult to warm a tumor at a deep part of a body selectively due to the electrical property of the living body, so that the satisfactory medical treatment result cannot be expected for a tumor at a part deeper than 5 cm. For this reason, for a treatment of a tumor at a deep part, a warming scheme using the ultrasonic energy having superior focusing and depth transmission properties has been proposed (Japanese Patent Application Laid Open No. 61-13955).
There has also been a report of a medical treatment method in which the above described hyperthermia treatment method is further developed to heat a tumor portion at a temperature over 80.degree. C. by focusing the ultrasonic waves generated by the piezoelectric transducer elements to a diseased portion so as to kill the tumor tissues instantaneously by causing the thermal degeneration (G. Vallancien et al., Progress in Urology, Vol. 1, pp. 84-88, 1991).
In this type of the medical treatment method, unlike the conventional hyperthermia treatment, there is a need to irradiate the ultrasonic waves thoroughly by scanning the focal point over the entire region in which the tumor exists, because very intense ultrasonic waves are going to be injected into a very localized region in a vicinity of the focal point. In particular, in a case of irradiating the ultrasonic waves as intense as several thousand W/cm.sup.2, the change of the acoustic characteristic due to the cavitation or the thermal degeneration of the diseased portion produced in conjunction with the ultrasonic wave irradiation is expected to pose a serious problem, and can possibly be a cause of the undesirable side effect, but the method of intense ultrasonic wave irradiation for resolving this problem has already been proposed by the present inventor and others (Japanese Patent Application No. 4-43604).
However, this type of the medical treatment method still has a drawback in that, as a very high energy is to be concentrated in a localized region, there is a possibility for causing the side effect such as a degeneration due to heat generation and a mechanical destruction of tissues when the acoustic characteristic changes at a tissue boundary surface of an organ or the like located in the ultrasonic wave passing region. This is particularly prominent at a coupling surface of an acoustic coupling agent of the ultrasonic wave generation source and a body surface of the living body, and in addition, the acoustic characteristics of the skin and the internal tissues are also expected to be different, so that there is a possibility for the side effect such as a burning to be caused on the organ in the ultrasonic wave passing region or the body surface by the intense ultrasonic wave irradiation. In fact, the degeneration of the body surface due to the ultrasonic wave Irradiation over an extended period of time had been confirmed in an animal experiment conducted by the present inventor and others.
Consequently, there appears to be a need for optimally controlling the irradiation intensity and time of the intense ultrasonic waves at the tissue boundary surface, but such an optimum control scheme has not been available heretofore.